Recent growth in the demand for economical and high traffic volume optical fiber communication systems has encouraged the development of small, high-speed, and low-cost optical modules. For this reason, there is an on-going effort to provide improvements in the integrated circuit (IC) that drives and controls laser diodes optical power. This would allow smaller optical modules that are capable of functioning with smaller input signals.
FIG. 1 shows a conventional laser diode driver 100. The laser diode driver 100 is in the form of a field effect transistor (FET) differential pair configuration. The laser diode driver 100 includes a first transistor 110 and a second transistor 112 having their respective drain nodes coupled and modulated by a current source 120 having a current Imod. Each of the respective collector nodes of the first transistor 110 and the second transistor 112 is connected to a high reference supply voltage (Vcc) through resistors 130 and 132 respectively. The laser diode driver drives a laser diode 150 the output signal of which is monitored by a photodiode 160.
During operation, differential input data signals (IN+ and IN−) are provided from a pre-driver amplifier 180 to the respective gate nodes of the first and second transistors 110 and 112 respectively. The current IMOD provided by the current source 120 is typically modulated in synchronization with the voltage waveform of the input data signal. In response to the differential IN+ and IN− signals, the differential pair configuration acts as a differential amplifier that provides a modulated output signal from the collector node of the second transistor 112 to the laser diode 150. As a result, the laser diode 150 generates an optical output data signal that is representative of the differential IN+ and IN− signals. In addition, during operation, a current source 122 delivers a continuous fixed bias current IBIAS to the laser diode 150. This is usually necessary to ensure proper laser dynamic performance and is commonly termed the “pre-bias” current.
In some applications the laser diode driver 100 has to address different conditions at one fell swoop. One example of such an application is a burst mode transmitter installed in the optical network units (ONUs) of a passive optical network (PON). Such a transmitter should be able to operate over a wide temperature range and support high speed bit rate as well as high sensitivity to current changes. To satisfy the above conditions by the laser diode driver 100, the current of the output drive signal is typically required to be at a high level to properly drive the laser diode 150. Furthermore, the rise time (tr) and fall time (tf) of the waveform transitions of the modulated output drive signal should be very short (an order of several tens of picoseconds). The tradeoff for satisfying these conditions is the disability to provide a wide dynamic range for current sources 120 and 122 for varied values of IMOD and IBIAS as well as the existence of overshot signals. Moreover, it is known that the relationship between the driver's 100 output signal and the optical output of the laser diode 150 changes significantly with the environmental temperature as well as between different laser diode types and even with the same laser type but different components.
It would be, therefore, advantageous to provide a solution that would enable the efficient and stable operation of laser diode drivers.